What our wastewater can tell us

Fret not about potty talk when asking researcher Carsten Prasse about an emerging subfield of epidemiology.

"Don't worry, you can talk to me about feces all day long," says Prasse, a Johns Hopkins assistant professor of environmental health and engineering. He goes on to explain that our wastewater—yes, the sewage that makes its way from toilets to treatment plant—contains important biomarkers that can tell researchers about a community's diet, drug intake, and even the presence of disease.

Wastewater epidemiology dates back to the early 2000s and until recently has largely been used abroad to monitor communities' use of illicit drugs. By collecting a pooled sample of a geography's wastewater, epidemiologists are looking not to an individual person's drug use but rather to overarching trends—whether a city's nicotine use is up or down, for example, or when fentanyl is first detected in a region. It's a powerful tool because it doesn't rely on accurate self-reporting, he says.

"Surveys might not be as effective because people are not reporting or are underreporting, or not participating at all," he says. "This is an unbiased approach." Armed with accurate data, experts are better equipped to develop customized public health interventions, he says.

In 2020, epidemiologists who were scrambling to track the spread of COVID-19 saw a new use for waste­water testing. By collecting samples from a wastewater treatment facility—or even farther upstream at the level of a neighborhood—researchers could potentially detect coronavirus outbreaks before patients exhibit symptoms. In one notable case, the University of Arizona made headlines last summer when it detected the presence of COVID-19 in a dormitory by screening the sewage from each building. University officials were able to quickly test all 311 residents and isolate two asymptomatic positive cases, effectively stopping an outbreak before it happened.

"What Arizona is doing totally makes sense because they're also getting trends" in addition to detecting individual outbreaks, Prasse says. "They're generating these charts where you can see the number of virus particles detected from sampling time to sampling time. And the advantage to that is, let's say you're in a dorm—it doesn't matter if you detect one particle or 100 particles. If there was nothing there before, this shows you there's something there, which means there must be an infected person."

When Johns Hopkins' Homewood campus reopened this spring for in-person classes, environmental health and engineering Professor Kellogg Schwab helped the university add wastewater testing to its arsenal of outbreak prevention tools, alongside social distancing and frequent saliva testing. Schwab and his team rely on a series of autosamplers, devices about the size of an Igloo cooler that tap into the sewer lines of each residence hall on campus. Every hour or so, a pump on the autosampler sucks a small sample into a sealed system. At the end of the day, the researchers have what's called a composite sample, an accurate representation of everything that flowed through the pipes over the course of the day. From there, the sample is concentrated, purified, and then tested for SARS-CoV-2 using the same Reverse Transcription-PCR test test you might find at a drive-through testing site.

The hope is that wastewater testing can not only detect outbreaks in individual residence halls but also warn officials of a new wave of infections, alert them to new virus mutations, and help direct the best use of limited resources.

"This pooled approach could be a tool for us to say, 'Where do we really need to have a focused area where everyone needs additional testing?' Or, 'Our intervention is working pretty well; our baseline's working,'" Schwab says. "Nothing's absolute; don't get me wrong. … It's a powerful tool, but it's not a silver bullet."

Researchers caution that waste­water epidemiology, whether used to monitor illicit drug use, COVID-19, or even a community's consumption of green vegetables, is still a new and imperfect process. Among the considerations researchers must take into account: A factory or cancer clinic upstream could lead to higher than normal levels of pharmaceuticals in the wastewater. Chemicals in the wastewater could interact with other compounds and degrade as they make their way toward the treatment plant. And a heavy rain event could impact the concentration of a particular biomarker in the sample. Prasse also points out that in the case of COVID-19, each person may shed a different amount of the virus— spanning from superspreaders to those who are seemingly immune despite repeated exposure.

"When I talk about this technology, I always tell people there is still a lot of uncertainty involved," he says. "These are all things that we will need several years to address."